Hydromedusae
General morphology

The planktonic stages of the Hydroidomedusae or hydromedusae are free swimming, solitary animals, with essentially tetramerous radial symmetry (Radial symmetry). Their body, swimming bell, or umbrella has the general form of a mushroom, bell, or disk (Medusa, Medusa 2, Side view P. rubiginosa). Most of its volume is occupied by a gelatinous mass, the mesoglea, the jelly of the jellyfish, which confers shape and buoyancy (Medusa, Side view P. rubiginosa). The convex upper (aboral) umbrellar surface is called the exumbrella (Medusa, Medusa 2), and the concave lower (oral) surface is termed the subumbrella (Medusa, Medusa 2). The space enclosed by the umbrella is the subumbrellar cavity, which is narrowed by a muscular horizontal diaphragm, or velum, leaving only a central circular aperture, the velar opening (Medusa, Medusa 2, Closed statocyst, Bell margin, Bell margin 4). The velum plays an important role in the locomotion of the medusae. The free rim of the umbrella bears marginal tentacles and sense organs (ocelli, statocysts, cordyli: Medusa, Closed statocyst, Cordylus, Bell margin, Ocelli). The tentacles may be solid (Bell margin 4) or may contain an extension of the circular canal (tentacular cavity: Medusa). The base of each tentacle is commonly swollen into the tentacular bulb (Medusa 2, Bell margin, Bell margin 2, Bell margin 3, Flexile marginal cirri).

The manubrium, a tubular or quadrangular projection of variable length, hangs from the centre of the subumbrella as the clapper of a bell (Medusa, Medusa 2, Side view P. rubiginosa). The cavity of the manubrium, or gastric cavity, opens distally into the mouth (Medusa, Medusa 2) and extends proximally into the radial gastrovascular canals (Medusa, Medusa 2). These canals are generally 4 in number but are sometimes more numerous. Through the mesoglea they connect the gastric cavity to the circular canal, which runs all along the marginal rim of the umbrella (Medusa, Medusa 2, Bell margin 3). The gastric cavity, the radial canals, the ring canal, and the tentacular canals (when they exist) form the gastrovascular system, which serves both for digestion and for distribution of food, waste, cnidoblasts, and even gametes.

The mouth may be simple or provided with lips, lobes, or tentacles (Mouth structures). The radii corresponding to the radial canals are termed perradii. The interradii lie intermediate between them, and the adradii midway between the perradii and the interradii (Radial symmetry).

The "cathamnal" or endodermal lamella, a unistratified membrane crossing the mesoglea, interconnects the radial canals and, like them, connects the gastric cavity with the circular canal. It delimits two mesogleal levels, one subumbrellar, the other exumbrellar (Medusa, Medusa 2). The sex cells may develop and mature either on the manubrium, or the radial canals, or both. Fertilisation is usually external, but internal fertilisation may occur in a few species. The resulting embryo develops into a planula larva, which settles and metamorphoses into a new polyp stage, or more rarely directly into a new medusa.